Sea cucumbers are one of the weirdest-looking, grossest animals around. This post is not about sea cucumbers, but I promise there will be one on them in the future. No, this post is about a fish that chooses to live inside sea cucumbers. That’s one poor life decision.

Atlantic pearlfish are the wonderful fish that choose to live inside sea cucumbers. They can be found in the Atlantic (big surprise there!), from Brazil to Bermuda. They like warm waters, especially those around coral or shallow grassy areas, which I assume is also where sea cucumbers live.

In order to fit inside a sea cucumber, you have to be reasonably small, and Atlantic pearlfish are, reaching lengths 15 to 24 centimetres. Their dorsal fins run all the way along their body, meeting  the fish’s tails fins to give the fish a bit of an eel-like experience. Pearlfish are usually silver and white with some red markings on them.

Kind of neat looking, aren’t they? Image by Etrusko25, CC BY-SA 3.0, via Wikimedia Commons

Pearlfish can communicate via sounds, releasing pulses from inside their sea cucumber hosts. The sounds tell other pearlfish what sex they are, and can be produced by either the fish rubbing their body parts together or from vibrations of their swim bladders.

Atlantic pearlfish lay their eggs in a jellied mass that floats around the ocean until the eggs hatch. There are two phases that baby pearlfish must undergo before adulthood, the vexillifer stage and the tenuis stage. Tenuis pearlfish are long and thin, and see-through. They enter a sea cucumber through its anus, and get in tail first. That must be one awkward thing to watch.

The pearlfish find hosts by swimming around until they feel water currents exhaled from unwary sea cucumbers. Once a pearlfish has entered a cucumber, it will either remain inside the sea cucumber’s stomach or will bite through to other areas of the poor hosts’ body. I would definitely not want to be a sea cucumber targeted by a pearlfish. Though some species of pearlfish are parasitic to their hosts, the Atlantic pearlfish aren’t, using the sea cucumber for shelter only. They leave their hosts at night to feed, and reenter in the day to stay safe.

Though spending your life inside the stomach of a sea cucumber is probably one of the grossest things ever, it is a pretty amazing life strategy. I would never have thought of doing something like that, but then again, I’m not evolution.

Cover image by Etrusko25, CC BY-SA 3.0, via Wikimedia Commons